Let us derive the usual elastic equation
from the lattice model with the nearest-neighbor interaction
with coupling constant \(g_2=K\)  by the method suggested in
[1]}, [2]}, [3]}.
We will use equations (REF ) with
\(\sum _{\begin{array}{c}m=-\infty \\ m \ne n\end{array}}^{+\infty } \; K_2(n,m) \; u_m(t) =u_{n+1}(t)-2u_n(t)+u_{n-1}(t) , \quad K_4(n,m)=0 ,\) 
